Patients with Smith-Lemli-Opitz syndrome have a cholesterol deficiency because there is a block in the synthesis of cholesterol. Clinically, they have microcephaly, mental retardation and hypotonicity, along with very low plasma cholesterol levels and high concentrations of 7-dehydrocholesterol. the studies involve feeding dietary cholesterol to alleviate the deficiency and to block biosynthesis of 7-dehydrocholesterol. Cholesterol absorption, the sterol balance and turnover, bile acid synthesis and turnover will all be measured under conditions of cholesterol-free and high cholesterol diets.